Partial fingerprint scanners are becoming popular for a wide variety of security applications. In contrast to “all at once” fingerprint scanners, which capture an image of an entire fingerprint at the same time, partial fingerprint sensing devices use a sensing area that is smaller than the fingerprint area to be imaged. By imaging only a portion of a fingerprint at any given time, the size and cost of a partial fingerprint sensor can be made considerably smaller and cheaper than that of a full fingerprint sensor. However to capture a full fingerprint image, the user must move his finger and “swipe” it across the sensing zone of the partial finger print sensor.
Various types of partial fingerprint readers exist. Some work by optical means, some by pressure sensor means, and others by capacitance sensing means or radiofrequency sensing means.
For example, one common configuration used for a fingerprint sensor is a one or two dimensional array of CCD (charge coupled devices) or C-MOS circuit sensor elements (pixels). These components are embedded in a sensing surface to form a matrix of pressure sensing elements that generate signals in response to pressure applied to the surface by a finger. These signals are read by a processor and used to reconstruct the fingerprint of a user and to verify identification.
Other devices include one or two dimensional arrays of optical sensors that read light reflected off of a person's finger and onto an array of optical detectors. The reflected light is converted to a signal that defines the fingerprint of the finger analyzed and is used to reconstruct the fingerprint and to verify identification.
Many types of partial fingerprint scanners are comprised of linear (1 dimensional) arrays of sensing elements (pixels). These one dimensional sensors create a two dimensional image of a fingerprint through the relative motion of the finger pad relative to the sensor array.
One class of partial fingerprint sensors that are particularly useful for small device applications are deep finger penetrating radio frequency (RF) based sensors. These are described in U.S. Pat. Nos. 7,099,496; 7,146,024; and U.S. Publication Nos. US2003-0035570 A1; US2004-0081339 A1; US2005-0244038 A1; US 2005-0244039 A1; US2006-0083411 A1; US2007-0031011 A1, and the contents of these patents and patent applications are incorporated herein by reference. These types of sensors are commercially produced by Validity Sensors, Inc, San Jose Calif. This class of sensor mounts the sensing elements (usually arranged in a one dimensional array of conducting electrical traces) on a thin, flexible, and environmentally robust support, such as Kapton tape, and the IC used to drive the sensor is mounted in a protected location some distance away from the sensing zone. Such sensors are particularly advantageous in applications where small sensor size and sensor robustness are critical.
The Validity fingerprint sensors measure the intensity of electric fields conducted by finger ridges and valleys, such as deep finger penetrating radio frequency (RF) based sensing technology, and use this information to sense and create the fingerprint image. These devices create sensing elements by creating a linear array composed of many miniature excitation electrodes, spaced at a high density, such as a density of approximately 500 electrodes per inch. The tips of these electrodes are separated from a single sensing electrode by a small sensor gap. The electrodes are electrically excited in a progressive scan pattern and the ridges and valleys of a finger pad alter the electrical properties (usually the capacitive properties) of the excitation electrode—sensing electrode interaction, and this in turn creates a detectable electrical signal. The electrodes and sensors are mounted on thin flexible printed circuit support, and these electrodes and sensors are usually excited and the sensor read by an integrated circuit chip (scanner chip, driver chip, scan IC) designed for this purpose. The end result is to create a one dimensional “image” of the portion of the finger pad immediately over the electrode array and sensor junction.
As the finger surface is moved across the sensor, portions of the fingerprint are sensed and captured by the device's one dimensional scanner, creating an array of one dimensional images indexed by order of data acquisition, and/or alternatively annotated with additional time and/or finger pad location information. Circuitry, such as a computer processor or microprocessor, then creates a full two-dimensional fingerprint image by creating a mosaic of these one dimensional partial fingerprint images.
Often the processor will then compare this recreated two dimensional full fingerprint, usually stored in working memory, with an authorized fingerprint stored in a fingerprint recognition memory, and determine if there is a match or not. Software to perform fingerprint matching is disclosed in U.S. Pat. Nos. 7,020,591 and 7,194,392 by Wei et. al., and is commercially available from sources such as Cogent systems, Inc., South Pasadena, Calif.
If the scanned fingerprint matches the record of an authorized user, the processor then usually unlocks a secure area or computer system and allows the user access. This enables various types of sensitive areas and information (financial data, security codes, etc.), to be protected from unauthorized users, yet still be easily accessible to authorized users.
The main drawback of partial fingerprint sensors is that in order to obtain a valid fingerprint scan, the user must swipe his or her finger across the sensor surface in a relatively uniform manner. Unfortunately, due to various human factors issues, this usually isn't possible. In the real world, users will not swipe their fingers with a constant speed. Some will swipe more quickly than others, some may swipe at non-uniform speeds, and some may stop partially through a scan, and then resume. In order to account for this type of variation, modern partial fingerprint sensors often incorporate finger position or motion sensors to determine, relative to the partial fingerprint imager, exactly where on the fingerprint a particular partial fingerprint image comes from, and how the overall finger position and speed varies during a finger swipe. A finger position indicator can be used to derive finger motion and acceleration, and in this document, finger position indicators, finger motion (or movement) indicators, and finger acceleration indicators are used interchangeably.
One type of finger position/motion indicator, represented by U.S. Pat. No. 7,146,024, and U.S. Publication Nos. US2005-0244039 A1 and US2005-0244038 A1 (the contents of which are incorporated herein by reference) detects relative finger position using a long array of electrical drive plate sensors. These plates sense the bulk of a finger (rather than the fine details of the fingerprint ridges), and thus sense the relative position of the finger relative to the linear array used for fingerprint sensing. A second type of fingerprint position indicator, represented by U.S. Publication No. US2007-0031011 A1 (the contents of which are incorporated herein by reference), uses two linear partial fingerprint sensors, located about 400 microns apart. The two linear sensors use the slight timing differences that occur when a fingerprint swipe first hits one sensor and then the other sensor to detect when a fingerprint edge passes over the sensors. This technique can also detect relative speed of passage over the two partial sensors. This type of information can be used to deduce overall finger location during the course of a fingerprint swipe.
In either case, once finger position is known, each of the one-dimensional partial fingerprint images can then be annotated with additional (and optional) time data (time stamp) and/or finger (finger tip, finger pad, fingerprint location) location data (location stamp). This optional annotation information, which supplements the “order of data acquisition” that would normally be used to keep track of the multiple stored partial fingerprint images in memory, can be used to help to correct distortions (artifacts) when the various one dimensional partial images are assembled into a full two dimensional fingerprint image.
Although finger location or movement sensors are usually adequate to track finger motion in the middle of a finger swipe, such sensors often encounter difficulty near the end of a finger swipe. For example, consider the situation where a finger is swiped over a finger motion detector that consists of a series of plates, and the user swipes the finger using a motion that brings the tip of the finger towards the user (see FIGS. 1 and 2). During the early part of the swipe, the finger will rest on many plate detectors, and changes in the finger's position will be reported by many of these detectors. However near the end of the swipe, as the tip of the finger passes the final series of plate motion detectors, only a few plates now can report a finger motion signal, and these plates may tend to report noisier and less reliable finger location and motion information. Since high quality finger position data is needed in order to properly determine what portion of the fingertip is actually being monitored by the partial fingerprint imager at a given time, the position of the last few partial fingerprint images towards the end of the swipe can't be as precisely identified, and thus this data is of less utility in constructing an overall image of the fingertip.
Similarly, near the ends of a finger swipe, other types of finger location or motion detectors can also return inadequate finger position and velocity data. For all types of finger location or motion sensors, the consequence of misreported or non-reported finger position and motion data means that not all of the partial fingerprint images (particularly those obtained near the end of the swipe) can be localized to an accurate location on the user's fingerprint. Since most of these partial fingerprint imagers return only single line (one dimensional) partial fingerprint images, as might be imagined, such one dimensional images are of marginal or no value if their precise location on the fingerprint (e.g. their “Y” axis) can't be identified.
As a result, prior art partial fingerprint imaging systems (in particular the algorithms and computational parts of these systems) tended to cope with such problems by simply truncating the reported fingerprint image once the finger position data started to become less accurate. Often this meant that these prior art systems ended up completely discarding the partial fingerprint images from the edge of the fingerprint, such as from the fingertips.
This loss of fingerprint data was both unfortunate and suboptimal. Fingerprints are typically compared to a database of other fingerprints by some sort of pattern recognition process. As in all pattern recognition challenges, more data is better, and fingerprint analyzers that more efficiently and accurately report a larger portion of a fingerprint will tend to produce superior results in pattern recognition applications. There will be fewer mismatches, and a greater chance of successful matches.
Thus there is a need in the art for partial fingerprint systems that can report a larger portion of a user's fingerprint. In particular, there is a need for superior methods to analyze the data reported by partial fingerprint scanners and finger position sensors. As will be seen, the invention accomplishes this in an elegant manner.